DOI: https://doi.org/10.36347/sjmcr.2026.v14i05.055

Pages: 1101-1104

Background: Primary retroperitoneal neuroendocrine tumors are exceptionally uncommon and should be diagnosed only after careful exclusion of a more frequent metastatic or pancreaticoduodenal origin. Their association with synchronous papillary thyroid carcinoma is rarely reported and may complicate diagnostic staging and therapeutic sequencing. Case presentation: A 53-year-old man with no relevant medical history other than medically treated L4-L5 disc herniation presented with isolated left lumbar pain. Thoraco-abdomino-pelvic computed tomography revealed a retroperitoneal mass initially suspected to arise from the pancreatic uncinate process. Magnetic resonance imaging showed a retroperitoneal lesion encasing the superior mesenteric artery with associated satellite lymphadenopathy. Computed tomography-guided biopsy confirmed a well-differentiated neuroendocrine tumor, World Health Organization grade 1. Further staging with 18F-fluorodeoxyglucose positron emission tomography/computed tomography demonstrated mild metabolic activity of the retroperitoneal lesion and an incidental right thyroid hypermetabolic focus. Upper gastrointestinal endoscopy and total colonoscopy did not identify a digestive primary tumor. Cervical ultrasound classified a 9 x 7 mm right thyroid nodule as EU-TIRADS 5, and fine-needle aspiration cytology confirmed papillary thyroid carcinoma. After multidisciplinary discussion, total thyroidectomy with lymph node dissection was planned, while somatostatin receptor imaging was scheduled to complete neuroendocrine tumor characterization and guide subsequent management. Conclusion: This case emphasizes the diagnostic complexity of retroperitoneal neuroendocrine tumors and the value of comprehensive multimodal staging. Synchronous papillary thyroid carcinoma, although unusual, should be recognized because it influences treatment prioritization, follow-up strategy, and long-term oncologic surveillance.